As a result of continuous technological advances that have brought about remarkable performance improvements, light-emitting diodes (LEDs) are increasingly finding applications in traffic lights, automobiles, general-purpose lighting, and liquid-crystal-display (LCD) backlighting. As solid state light sources, LED lighting is poised to replace existing lighting sources such as incandescent and fluorescent lamps in the future since LEDs do not contain mercury, exhibit superior longevity, and require low maintenance.
A light-emitting diode (LED) is a semiconductor device that emits light when its p-n junction is forward biased. While the color of the emitted light primarily depends on the composition of the material used, its brightness is directly related to the current flowing through the junction. As a result, an effective way to ensure that LEDs produce similar light output is to connect them in series so that all LEDs in the string have the same current. Unfortunately, a major drawback of the series connection of LEDs is the cumulative voltage drop that eventually limits the number of LEDs in a string. This limitation can be overcome by paralleling LEDs or LED strings. However, since the voltage-current characteristic (V-I curve) of individual LEDs differ and because the LED's forward-voltage drop exhibits a negative temperature coefficient, paralleled LED strings may not have the same, or even similar, currents unless a current sharing (balancing) mechanism is provided.
Generally, current balancing of LED strings connected in parallel can be achieved by a number of techniques. FIG. 1 shows a prior art current balancing approach achieved by connecting current-limiting resistors R1 to Rn in series with corresponding LED strings. While this approach offers simplicity and low cost, its performance is very limited. Specifically, the current balancing accuracy of this passive method solely depends on the matching of the LED string voltages and tolerances of the current-limiting resistors. Generally, current balancing performance of this approach is poor since LED string voltages exhibit significant differences primarily due to manufacturing tolerances and temperature variations.
FIG. 2 shows another prior art method of load current balancing with current regulators. In this method, the current in each LED string is independently regulated by a corresponding current regulator. As a result, the current in each string can be set precisely to the desired current. Generally, the current regulators can be linear or switching type. Switching regulators offer better efficiency than linear regulators and can be implemented with a step-up and/or step-down topology, making it possible to drive a variety of LED strings, including those with string voltages higher than the source voltage. One the other hand, linear current regulators are more cost effective than their switching counterparts. The major disadvantage of this approach is its implementation cost is relatively high, especially in applications with a large number of paralleled LED strings, because it requires a current regulator for each string.
FIG. 3 shows another prior art method that provides excellent current balancing with a reduced cost compared with the method in FIG. 2. In the approach, disclosed in U.S. Patent Application No. 2009/0195169 by Chung-Tsai Huang et al, current balancing transformers are employed to equalize the currents of the LED strings. In the three-string implementation of the magnetic balancer, as shown in FIG. 3, two transformers with an equal number of turns of the primary and secondary windings are connected between the output rectifier and the filter capacitor in the three isolated outputs of the converter. The current feedback from one output is used to set and regulate the current of the corresponding LED string. Because of the 1:1 turns ratio of the transformer windings, the current flowing through one winding of the transformer produces substantially the same current flowing through the other winding of the transformer provided that the magnetizing current of the transformer is small compared to the winding current. Therefore, if the current of string S3 is regulated by a feedback control as illustrated in FIG. 3, the current of string S2 will be equal to that of string S3 because the currents flowing through windings W3 and W4 of transformer TR2 will be equal. Because the current of string S2 also flows through winding W2 of transformer TR1, the current flowing through winding W1 of transformer TR1, i.e., the current flowing through string S1, will also be equal to that of strings S2 and S3.
A major deficiency of this cost-effective and high-performance magnetic current balancer is that it needs to be integrated with a switch-mode power supply, i.e., the current balancer cannot be used independently. As a result, this approach lacks the flexibility to operate with an arbitrary DC source, for example, a DC battery. In addition, the integration of the magnetic balancer into a switch-mode power supply increases the complexity and, therefore, the cost of the power supply because it requires a separate output for each string. Requiring separate outputs is especially detrimental in applications with a large number of paralleled LED strings.
Therefore, the need exists for a cost-effective and high-performance current balancer that can operate from any DC source.